1. Field of the Invention
The present invention relates generally to the fields of immunology and molecular biology. More particularly, it concerns improved protective and immunotherapeutic vaccines.
2. Description of Related Art
The most frequently used promoters in the production of a genetic vaccine for humans are viral in origin. Examples of such viral promoters include but are not limited to Mouse Mammary Tumor Virus (MMTV) promoter, Human Immunodeficiency Virus Long Terminal Repeat (HIV LTR) promoter, Moloney virus, avian leukosis virus (ALV), Cytomegalovirus (CMV) immediate early promoter/enhancer, Rous Sarcoma Virus (RSV), adeno-associated virus (AAV) promoters; adenoviral promoters, and Epstein Barr Virus (EBV) promoters. One of the drawbacks of viral promoters is their tendency to be silenced when there is an immunological response. It has been shown that many commonly used viral promoters are silenced by interferons and TNF-alpha at the level of mRNA stability (Qin et al., 1997). Compatibility of viral promoters with certain antigens appears to affect gene expression levels of these promoters. Other factors that appear to affect the level of gene expression are introns (e.g. CMV intron A), as well as, polyadenylation sequences such as Bovine Growth Hormone (BGH) or SV40 versions.
Of the regulatory elements known to the skilled artisan, the cytomegalovirus (CMV) immediate-early promoter/enhancer, is the most commonly used in gene therapy and in genetic (DNA-based) vaccines. The CMV promoter exhibits strong activity in various cell lines in vitro. In genetic vaccines, the CMV promoter drives a high level of gene expression and can successfully elicit protective immune responses in various animal hosts. However, one of the common problems encountered by investigators is that transgenes are expressed only at a low level and only transiently. Another major caveat of the CMV promoter lies in the large amount of plasmid DNA needed for intramuscular injection in primates (including humans) to render it effective for the purpose of genetic immunization. Although the molecular mechanisms that are responsible for poor expression of the CMV promoter in primates are poorly defined, it could be mainly due to attenuation of the promoter. Thus, in current approaches utilizing CMV promoter/enhancer elements, genetic vaccines have proven to be inefficient in primates. Therefore, a more efficient promoter is required to circumvent the deficiencies of the CMV promoter as a genetic (DNA-based) vaccine.
DNA vaccines represent an emerging field that relates to the art of preventing and treating disorders, diseases, conditions and infections by inducing immune responses in individuals directed at antigens associated with such disorders, diseases, conditions and infections. Specifically, plasmid DNA and linear and circular elements that include coding sequences for antigens operably linked to regulatory elements required for gene expression is administered to individuals. It is noted that the antigens need not be physically linked to the regulatory elements. The cells of the individual take up the plasmid DNA and the coding sequence is expressed. The antigen so produced becomes a target against which an immune response is directed. The immune response directed against the antigen provides the prophylactic or therapeutic benefit to the individual against any allergen, pathogen, cancer cell or autoimmune cell that includes an epitope which is recognized by the immune response against the antigen.
DNA vaccines include naked and facilitated vaccines. Further, they may be administered by a variety of techniques that include several different devices and compositions for administering substances to tissue. The published literature includes several review articles that describe aspects of DNA vaccine technology (McDonnel et al., 1996; Robinson. 1995; Fynan et al., 1995; Pardoll et al., 1995; Spooner et al. 1995).
Although CMV DNA-based vaccines could be effective in eliciting an immune response in some cases, their major drawback lies in their diminished efficacy in some hosts, particularly humans. Therefore, there is a need for compositions and methods that produce an enhanced and effective immune response.